1. Field of the Invention
The present invention relates to a method for smoothing an electrode disposed on a base, such as a substrate, a method for manufacturing a ceramic substrate using the smoothing method, and a ceramic substrate manufactured by the manufacturing method.
2. Description of the Related Art
In general, sintered metal electrodes disposed on ceramic substrates have high surface roughness and therefore do not necessarily have sufficient smoothness. In particular, electrodes that are formed on a ceramic substrate manufactured by the following process are likely to have insufficient smoothness: a non-shrinkage process in which the ceramic substrate is fired in a firing step so as not to transversely shrink such that constraining layers made of a sintering-resistant material that is not substantially sintered at the firing temperature of the ceramic substrate are provided on principal surfaces of the unfired ceramic substrate. This is because the electrodes have surface irregularities formed when removing the constraining layers.
Furthermore, electrodes formed by a screen-printing process or other processes are likely to have high surface roughness due to mesh traces.
Electrodes arranged on ceramic substrates are used as junctions for, for example, other substrates or mounting lands of circuit components. However, there is a problem in that the electrodes are likely to have insufficient properties, such as platability and wettability, when the electrodes have high surface roughness.
In a case in which, for example, electrodes formed on ceramic substrates are subjected to Ni plating, Ni particles grow isotropically. When these electrodes, which are bases, have irregularities, secondary particles of the Ni particles continue to grow so as to gradually cover the Ni particles deposited in recessed portions, and therefore, the irregularities are smoothed to some extent. Since the time required for the secondary particles to reach a certain height varies depending on the locations thereof, boundaries of the secondary particles grow with the growth of secondary particles. As a result, macroscopic networks of impurities are formed at the secondary particle boundaries. The impurities are melted during displacement plating to cause significant local corrosion. This causes blackening of the electrodes which causes problems, such as, a reduction in the soldering contact area and solder popping caused by water that is trapped in corroded portions.
Where electrodes that are subjected to wire bonding have surface irregularities, the wire pull strength thereof is relatively low. When such an electrode is subjected to Ni or Au plating, a Ni or Au plating layer growing thereon follows irregularities of the electrode and grain boundaries and therefore has surface irregularities.
Where surface irregularities of an electrode is smoothed by forming a thick plating layer thereon, there is a problem in that a substrate having the electrode disposed thereon is damaged and has a reduced strength because the immersion time of the substrate in a plating bath is relatively long and a portion of the substrate is dissolved.
In view of the problems described above, the following method has been proposed: a method in which the surface roughness of an external electrode removed from a constraining layer is adjusted to a range of about 0.1 μm to about 6 μm such that the platability and/or wettability thereof can be secured (see Japanese Unexamined Patent Application Publication No. 2007-318173).
Japanese Unexamined Patent Application Publication No. 2007-318173 describes that the platability and/or wettability of the external electrode can be ensured such that the surface roughness of the external electrode removed from the constraining layer is adjusted to about 0.1 μm to about 6 μm.
However, Japanese Unexamined Patent Application Publication No. 2007-318173 discloses no technique to accurately adjust or control the surface roughness of the external electrode removed from the constraining layer.